It has previously been proposed to utilise a tanker as an off-shore oil production platform. The tanker is provided with a turret having a generally cylindrical body extending vertically through the hull which acts as a pivot about which the ship can weathervane. The turret is moored by a number of mooring lines known as cateneries which extend to the seabed to prevent turret rotation. Oil production risers extend from a well head on the seabed into the turret and the output from the risers is fed into tanks in the ship for temporary storage. An arrangement of this kind is shown in GB No. 2 150 517 in which a rotary turret is provided on its underside with an annular platform to which the cateneries are attached using an external winch hung over the stern of the ship. The turret is supported at its lower end by an annular bearing arrangement which comprises an annular track around the turret and bearing members mounted on the hull of the ship which fit slidably in the track. The top end of the bearing is supported by another, upper annular bearing.
A difficulty with this bearing arrangement is that it cannot accommodate readily the stresses that occur on the turret when the ship is moored. It will be appreciated that the weight of the moorings, turret and heaving of the ship, particularly in heavy seas, produces substantial loads between the turret and the hull. Axial loads are accommodated primarily by the lower bearing, but the axial loading produces bearing wear. Also, axial loading produces a shear stress on the lower bearing's members that are attached to the hull. The upper bearing is not designed to accommodate substantial axial loading. Further, interchange of the upper bearing cannot be readily facilitated.